Squeezing Out Opportunities for Citrus Ingredients

September 2001

Among the most popular fruits, citrus fruits lend their distinctive flavor to a wide variety of food products, as well as being the source of choice for the breakfast juices found on many U.S. tables. Not only do citrus juices and flavors make useful tools for product designers, other ingredients from citrus also have tremendous potential.
In the past, byproducts have provided a supplemental revenue source for citrus processors when faced with price fluctuations. More stable conditions, however, have deterred this broader view. “Most of the juice companies are focused on the beverage end of it and not so much on the ingredient potential,” says Bill Stinson, research scientist, Florida Department of Citrus, Lake Alfred, FL. “Because of the capital associated with the ongoing research, it’s been difficult to get the industry on board with this.”

Nevertheless, many useful ingredients currently come from citrus and researchers are pressing forward with efforts to squeeze new functionality from this fruit.

From the outside in
Citrus possess a unique, modular construction. Its many beneficial components are located in the parts that most consumers would throw away.

The outer skin, or flavedo, contains pigments that give each fruit its distinctive color. This exterior surface also is dotted with oil glands. Just underneath is the white portion of the peel, or the albedo. Under the albedo lie the familiar citrus segments. Each of these is made up of several membrane-encased juice vesicles. Each segment’s collection of juice vesicles is further surrounded by an outer membrane known as the lamella. In seeded fruit, the seeds are found toward the center of the segments. Beyond that, citrus fruits have a core made of a soft, spongy material that visually resembles the albedo.

The various peel layers and membranes in citrus make it challenging to prepare fruit pieces on the large scale required for manufacturing use. Mechanical peeling and lamella removal is not 100% effective and often leaves membrane portions that — particularly in grapefruit — may contribute a bitter flavor. Chemical and enzymatic methods for peel and membrane removal often generate such low yields that processed citrus fruit frequently is sectioned by hand.

Sectioned pieces are packed in syrup and either canned in drums or frozen for large-scale users. Heat treatments reduce the toughness of any residual membrane, as well as enzymatic activity.

In addition to sectioned pieces, isolated whole juice vesicles also are available. These pieces’ naturally small size makes them a good choice for products that must undergo pasteurization, such as yogurt.

A commercial process for creating whole juice vesicles first appeared in the late 1940s. Although researchers have refined and improved this process over the years, often hand-labor still is required to achieve acceptable yield. Of course, “acceptable” is relative, given that more than half of the juice vesicles still are lost. To improve this, researchers have looked at several new methods. Some use high-pressure air or water to separate the vesicles from their membranous home. Another uses a rotating screen drum with a central shaft studded with metal protrusions that act like fingers to separate the vesicles. One of the more unique approaches uses cryogenic freezing.

First attempted in the late 1980s, researchers at the Florida Department of Citrus have revived the process to create a frozen version they’ve dubbed, “Citrus Pearls.” The researchers use liquid nitrogen to freeze the citrus fruit to -50°F. When the fruit is subjected to impact, it shatters into a handful of frozen vesicles. In addition to their potential as an ingredient, the Florida Department of Citrus is looking into commercializing the citrus pearls for consumer use — including extending the current 90-day freezer life. The department hopes the process can be patented so that it may be licensed to manufacturers and predicts that cryo-separated juice vesicles will be available in supermarkets by around 2003.

Pressing matters
Citrus fruits are about 50% to 60% juice. Not only does juice make up the majority of the fruit, it also is the largest consumer market for citrus — with orange juice the clear leader. Consumers can purchase orange juice as frozen concentrated orange juice (FCOJ), chilled orange juice from concentrate (COJ) and, more recently, chilled, not-from-concentrate orange juice (NFC). To save costs from shipping water, however, juice reconstitutors and other industrial-juice users typically purchase frozen concentrate.

Juice processing originated as a way to use fruit that was deemed unsuitable for the fresh market. Now, more than 80% of the oranges grown in Florida are pressed for juice. First, mature fruit is washed and graded before stainless-steel juice extractors press out the juice. After extraction, the juice passes through a finisher which, using various means, removes excess pulp and seeds.

For concentration, the finished juice flows into a specialized evaporator that will remove water. Different evaporators use various technologies — heat, vacuum or some combination thereof — to quickly increase the juice concentration from around 12° Brix to around 60° Brix. Minimizing heat exposure helps preserve the flavor, which is degraded by heat and oxygen exposure. Even with a vacuum system, some pasteurization heat still is required to kill spoilage microorganisms and to inactivate pectinase enzymes that cause juice solids to settle.

The concentrated juice isn’t frozen right away. Because fruit quality and flavor vary with the season, processors typically blend stored frozen concentrates to achieve consistency. The blended juice then is rapidly chilled, filled into steel drums or plastic-lined fiber containers and frozen. (Of course, juice for the retail market is packed in appropriate, smaller units.)

Food or beverage processors using frozen juice concentrate must thaw it. The more rapid the thawing, the better the quality, because oxygen is destructive to juice flavor components above freezing temperatures. Because of the high water content in lower Brix concentrates, they tend to freeze harder and require more thawing time — up to 24 hours for a 52-gal. container. The higher the concentrate’s Brix, the less solidly it will be frozen.

To speed up thawing, processors may employ various techniques. Some thaw drums just until the concentrate can be slid out and passed through a mechanical chopper. These choppers also may be jacketed to supply heat, but just enough to thaw the concentrate until it’s pumpable. Another method is to manually remove some of the partially thawed concentrate from the drum to a tank and let water and stirring finish the thawing process the way most consumers do.

Whatever, the method, product designers must consider thawing when creating products with citrus juice concentrate. Laboratory samples may be packaged in a different-size container from those used in the production facility. Differences in handling and thawing may lead to differences in flavor when the product is scaled up. If possible, use thawed concentrate obtained from the production facility if it’s nearby, or at least duplicate the eventual thawing process as much as possible.

To avoid the thawing step, manufacturers that use larger quantities of concentrate may purchase refrigerated bulk containers of pumpable concentrate. These must be stored below 15°F and blanketed with an inert gas, such as nitrogen or carbon dioxide, to minimize oxygen exposure.

This approach is similar to the technology that makes not-from-concentrate juices possible. Here, however, the entire system of storage tanks not only must be refrigerated and feature inert-gas blanketing, but be entirely sterile as well. The successful implementation of the technology is evident in the number of NFC products available to consumers.

Shifting the non-juice stream
Juice may be the primary component of citrus processing, but what becomes of the non-juice material? Some processors elect to do little more than find a cost-effective way of disposing of it. Often, the peel and other residue is treated with lime, pressed to remove fluids and dehydrated to around 10% for use as animal feed. Not only does the pressing reduce the energy required to dehydrate the mass, the resulting liquor is concentrated to form citrus molasses. Some processors add the molasses back to the peel, others sell it for beverage alcohol production.

Citrus is, however, capable of yielding much more. “Most of the waste material currently is used for cattle feed; I like to look at it as ‘non juice’ stream,” says Stinson. “The Florida Department of Citrus is looking to make that non-juice stream approach the use profile of the juice stream.”

One non-juice-stream example is pulp wash, the soluble solids extracted during a countercurrent water wash of the pulp stream from juice manufacturing. This can be concentrated and handled similarly to juice. Pulp wash concentrate may be added back to juice destined for FCOJ to help fill out flavor, or can be used to add fruit solids and natural cloudiness to juice drinks and other beverages. Even though it’s cheaper than juice concentrate, pulp wash concentrates usually are listed as “concentrated orange juice” on the product’s label.

Once washed of soluble solids, the insoluble pulp can be pasteurized and frozen. This can be used to add pulp content to various citrus-identified beverages. Because many of the flavoring components have been removed, the pulp may be used in products that aren’t necessarily the same flavor as the pulp’s source fruit. Some pulp also may be dried, ground and sold as dietary fiber.

Further from the fruit
During juice evaporation, both oil- and water-based flavoring compounds will flash off. These are captured via condensation and concentrated into essences. A primary use for essences is to add them back to the juice. This helps restore some of the flavor lost when the juice is exposed to the heat of evaporation. It also helps processors control the juice’s flavor strength. Essences also can flavor other beverages in addition to juice.

As mentioned earlier, citrus peel is dotted with oil glands. Water extraction of the peel yields an oil-and-water emulsion from which cold-pressed oils can be separated. Citrus oils naturally contain d-limonene that can generate off-flavors when it oxidizes. Concentrating oils via “folding” helps remove limonene and improve the stability and solubility of the oils.

Folding citrus oils involves vacuum distillation followed by an alcohol wash. As with juice processing, heating is minimized to protect the oil’s flavor. Some processors use other folding methods, too, such as liquid extraction, adsorption with activated silica, and extraction with supercritical carbon dioxide.

Product designers may choose from a range of concentrations for folded citrus oils. The more concentrated, the lower the potential for off-flavor development. In fact, 20-fold oils usually are given the designation “terpeneless.” Unfortunately, achieving these concentrations also may affect the flavor because flavor components may be lost during processing.

Although many processors in the food industry make use of citrus oils, the beverage industry is the largest user in the United States. Citrus oils also flavor candy, cookies, chewing gum and ice cream. In addition, oils and essences are key ingredients in citrus flavorings.

A citrus flavor may start with any of several fruit byproducts, including the peel, the essential oils and the essences. How these building blocks are assembled, however, can vary greatly. Some companies simply blend such ingredients with other flavorful substances using familiar flavor-compounding techniques. Others prefer to use specialized processing methods to emphasize the desired notes and de-emphasize any negative attributes without adding material that isn’t from the named fruit or chemically changing the starting material.

Such processes are particularly important for flavors designed to be added back to juices. Juice processing tends to damage certain flavor components. Adding an all-citrus flavor helps maintain consistency while keeping the product within standards of identity.

To maintain consistency, suppliers of these specialized citrus flavors must purchase raw material from worldwide sources. Not only does this ensure an ongoing supply, but by buying and blending from a variety of sources, the company also can provide ingredients with consistent flavoring properties.

“A lot of analytical work gets done in evaluating the condition of the fruit and the byproducts,” says Robert Kryger, innovation director for citrus, Danisco Cultor, Lakeland, FL. “We sometimes even work with our suppliers to increase the quality of the byproducts they’re producing.”

Advances in chemical testing methods and increasingly sophisticated fractionation technologies have helped these flavors become more specialized with higher quality. This allows from-the-named-fruit citrus flavors to be customized to a particular application much as a compounded flavor is. “You have to design a flavor with some sort of performance criteria,” says Kryger. “The best way is for the customer to be very specific as to the sort of product in which it’s going to be used, the packaging and the shelf-life requirements.”

When using citrus oils, essences and other citrus-based flavorings, it’s absolutely critical to minimize heat and oxygen contact. Whenever possible, they should be refrigerated and only ordered in quantities that will be used within a reasonable amount of time. To minimize contact with air, lab samples and partial containers in the production facility should always be stored with inert gas in the headspace.

An a-peel-ing thickener
Pectin is a polysaccharide that is present as protopectin in the flavedo, albedo and various membranes in citrus. As a water-binding hydrocolloid with gel-forming capabilities, it has many potential food-ingredient applications. Pectin is manufactured from spent citrus peels using acid extraction, precipitation and purification. Although orange peel may be used, limes, lemons and even grapefruit often provide higher yields.

Pectin’s exact molecular weight, pH and the number of methyl ester groups on its molecule all affect its functional properties. High-methoxyl (HM) pectins have a degree of methylation greater than 50%. They are soluble in water and gel in low-pH systems containing high sugar levels (greater than 60%). Low-methoxyl (LM) pectins have a degree of methylation less than 50%. They swell and hydrate in cold systems. LM pectins gel in the presence of calcium ions without any specific level of acid or sugar.

In addition to its familiar uses in jams and jellies, pectin also acts as a viscocifier and anti-settling agent in fruit juices and other beverages. At one time, it was offered as a part of a fat-replacing system.

Extracting pectin presents many manufacturing challenges. Perhaps the biggest hurdle is the environmental impact because the waste stream from pectin manufacturing has a high biochemical oxygen demand (BOD). “Not much pectin manufacturing goes on in the United States. Most of it comes from Europe,” says Stinson. “U.S. processors actually ship a rough, dried citrus — with the sugars, etc. removed — to Europe for that purpose.”

Stinson believes, however, that a U.S. manufacturer could exploit this potentially profitable opportunity. “Citrus is very high in pectin, it’s just a matter of removing it economically and with an environmentally aware process,” he says.

In addition to pectin, citrus peel also is a good source of fiber. Citrus fiber combines cellulosic and pectin-like materials. The proportions of these components vary depending on how the pulp, skin and albedo are isolated. The amount of total dietary fiber also varies by the season. Blending different lots controls consistency and specific content.

When extracting the fiber from peel, one of the challenges is minimizing bitterness contributed primarily by the naringin and limonin found in the flavedo. At the Florida Department of Citrus, Stinson is working on a citrus-based fiber ingredient that not only is less bitter, but offers useful functional properties.

“Its composition will be half soluble, half insoluble fiber,” says Stinson. “We think that because of that composition, it could be a very desirable product as an additive for products in which you want to increase total fiber content.”

Although work continues on improving the fiber’s blandness, Stinson’s team has performed a great deal of application work in the lab — particularly in the area of bakery foods. “When we bake bread with a small percentage of this fiber, it increases the bread’s moistness and extends its shelf life from a tactile standpoint, by five to seven more days,” he says. “We find the same thing in other forms of bakery foods. It tends to result in a moister cake, or moister cookie.”

Beyond bakery foods, the Department of Citrus has tested the fiber’s potential as an extender in hamburger patties, where the researchers discovered it helps retain juiciness. Such examples of citrus fiber’s water-retaining properties reinforce its potential as a fat replacer.

Researchers at the Miguel Hernández University in Spain also tested the idea by using lemon albedo as a fat replacer in cooked pork sausages. Presenting their results at this year’s Institute of Food Technologists Annual Meeting in New Orleans, the researchers reported how they added citrus fiber at 2.5% and 5.0% along with a control. They found that the citrus fiber decreased the sausages’ pH as well as reduced the residual nitrite level as citrus fiber increased in concentration. Color evaluation revealed that the addition of the fiber increased lightness, yellowness and hue, while reducing redness and chroma. Up until the 5.0% level, the researchers found no significant changes in the sausages’ sensory characteristics.

Healthful fiber functionality
In addition to functional benefits, past studies at the University of Florida and elsewhere indicate that citrus pectin and fiber have potential health benefits. Several studies have also shown, for example, that pectin can decrease serum cholesterol levels without effecting serum triglyceride levels. Pectin also can reduce blood sugar spikes when consumed with a meal. Still more research indicates pectin may reduce the risk of certain cancers. For example, researchers at the School of Medicine, University of Michigan, Ann Arbor, studied a pH-altered modified citrus pectin and found that it prevented spontaneous prostate cancer metastasis by inhibiting cancer cells from adhering to other cells in the body.

Other researchers in Texas also have discovered a link between pectin and reduced prostate cancer risk. Scientists at the Texas A&M-Kingsville Citrus Center at Weslaco, the University of Texas-Pan American at Edinburg and Texas A&M’s Institute of Biotechnology (IBT) at the Texas Medical Center in Houston collaborated on the research, which was published in the June 2001 issue of the Journal of Agriculture and Food Chemistry. As with the Michigan research, this study showed that citrus pectin somehow inhibits the mechanism that triggers prostate malignancy. The next step for the Texas researchers is to identify the active component of the pectin. Once that is done, they’ll explore ways to increase pectin consumption by enhancing pectin’s presence in citrus via modified growing and harvesting practices, or by extracting and modifying the active ingredient of pectin and making it available as an ingredient.

Benefits beyond vitamin C
Health benefits are nothing new to citrus fruit. For years, consumers have recognized not only the benefits of vitamin C, but citrus’ role as a source of the nutrient. As an antioxidant, vitamin C can protect LDL cholesterol from oxidation to help reduce the incidence of heart disease. It also can also block the formation of carcinogenic nitrosamines in the body. Vitamin C also enhances cellular immunity by promoting the killing ability of white blood cells.

In addition to vitamin C, however, citrus has many other components that offer health benefits. Some sources even claim that an orange contains more than 170 phytochemicals. These components generally fall into the categories of carotenoids, flavonoids, terpenoids, limonoids and glucarates.

Carotenoids. Oranges contain around 20 carotenoids, but only red grapefruit has a high beta-carotene content. Other carotenoids, such as lutein, zeaxanthin and beta-cryptoxanthin, can be found in significant quantities in tangerines and oranges. Although not beta-carotene, these other carotenoids also possess significant antioxidant activity and protect against age-related macular degeneration. Red grapefruit also contains a high level of lycopene. Among dietary carotenoids, lycopene has the highest antioxidant activity. Epidemiological studies at the Dana-Farber Cancer Institute of Harvard Medical School in Boston have shown that lycopene can produce a significant reduction in prostate cancer risk.

Flavonoids. Researchers have found more than 60 flavonoids in citrus. Flavonoids not only offer antitumor and antiviral activity, they can act as anti-inflammatory agents and help reduce the risk of coronary heart disease. Quercetin, for example, has a greater antioxidant activity than beta-carotene and vitamin E. Tangeretin and nobiletin both can inhibit tumor cell growth and trigger a body’s natural detoxifying enzymes.

Terpenoids. The very components that are undesirable in essential oils actually have healthful benefits. Orange and lemon oil contain substantial amounts of limonene, a terpenoid with anticancer properties.

Limonoids. Citrus fruits feature around 40 limonoids, limonin and nomilin predominating. These substances contribute to the bitter taste found in citrus — particularly in grapefruit and orange juice where they occur in high concentrations. Studies have shown that limonoids inhibit tumor formation in animals. They act by stimulating the major detoxifying enzyme, glutathione S-transferase. Laboratory tests with human breast cancer cells actually have shown that certain limonoids have greater antitumor activity than the anticancer drug tamoxifen.

Glucarates. These are found in the pulp and albedo of citrus. Current studies are underway to determine their potential for preventing breast cancer and easing the symptoms of premenstrual syndrome.

The Citrus Physiology and Nutraceutical Program and Texas A&M University (College Station), in collaboration with Vegetable and Fruit Improvement Center (College Station), The Institute of Biosciences and Technology (Houston) and Baylor College of Dentistry (Dallas), currently are seeking to enhance the nutraceutical content of citrus. Other researchers are focusing on ways to extract the beneficial compounds from the non-juice citrus processing stream. “It would be a tremendous opportunity if we could take the non-fibrous stream and further fractionate that to recover the various flavonoids and other beneficial chemicals, and sell them to either the supplement or the functional food markets,” says Stinson.

Recovering phytochemicals from non-juice citrus components typically involves extraction under specific conditions. Although researchers have developed methods to extract limonoid glucosides and other substances, methods for many of the more potent phytochemicals remain to be discovered.

Product developers always are seeking ways to improve a product’s performance, flavor and health contributions. The various materials obtained from citrus offer benefits in all of those areas. In addition, having citrus-based materials appear on the product label also carries a positive impression among consumers. Hopefully, more citrus processors will see this potential and create more useful ingredients from citrus.

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